The invention relates to electro-optical display devices of the type that can be rapidly switched from one visual state to another and of the type that possesses inherent memory to retain the display in any of its various display states for either an indefinite period or a selectively terminable interval of time. More particularly, the invention relates to such a display device having an electrically conductive polymer film on the display electrode(s) thereof and having one or more types of pigment molecules embedded in the polymer film to provide a plurality of display colors that contrast sharply with the background of the display area in the device and that can be very rapidly switched from one color to another.
Electrochromic display devices of various types are generally well known and have come into extensive use in products such as digital display watches and video game display panels. Typically, the display effect in such devices is achieved by changing the electric potential of a display electrode relative to a counter electrode in the device to cause a film on the display electrode to electrochemically change color. Such electrochromic display devices are superior to either the types of light emitting diode or plasma display panels that preceded them in development, because they require substantially less power to achieve the display function. While liquid crystal display devices have been developed with lower power requirements than those of light emitting diode or plasma display panels, they have other inherent disadvantages. For example, the visual effect achievable from liquid crystals is severely limited by the viewing angle, i.e. if viewed from an angle several degrees away from an axis orthogonal to the plane of the display surface the visibility of the display is significantly decreased. Also, liquid crystal displays have essentially no residual memory function within the liquid crystal materials.
In the earliest electrochromic display devices, a color change was typically effected between a single dark color and a white or yellowish color, but no other variations in color were available. Changes in color display states in those early devices were caused by a redox reaction that was controllably triggered responsive to the passage of an electric charge between the display and counter electrodes when electric potential was switched in the devices. Such electrode deposition mechanisms were relatively slow in both the write and erase modes and a significant amount of power was required to achieve them. The relatively long time of visual display transition, or switching time, and the power requirements of those early electrochromic display devices both served to limit their application, particularly where batteries were needed as a power source for the display.
Many of the disadvantages inherent in the earlier electrochromic display devices have been overcome by the development of display devices that utilize metal diphthalocyanine complexes as the active electrochromic material in the display cells. By the use of such complexes, the display switching times have been improved to effect color changes in less than 50 milliseconds, versus the 0.1 second switching times typically required for the earlier generation of display devices that used a redox reaction requiring the passage of a charge through an electrolyte to a display electrode. Also, the use of such complexes has resulted in the production of multi-color displays which are readily obtainable by applying a range of selected voltages across the display and counter electrodes of an electrochromic display cell having a certain kind of metal diphthalocyanine film covering its display electrode. Examples of such relatively recently developed electrochromic display devices are discussed in U.S. Pat. No. 4,184,751, which issued on Jan. 22, 1980. The display devices disclosed in that patent are characterized by utilization of an electrochromic film formed of a selected diphthalocyanine complex that is preferably deposited in a continuous layer over the display electrodes in the device and also over the nonconducting, electrode-free, regions between the display electrodes. However, the patent does suggest that it is possible as an alternative form, to deposit the electrochromic film in separate segments so that the areas between the display electrodes are not covered by the film. The observed advantage of using such separate segments of the electrochromic film is that visual distraction caused by retention of the original film color in the regions between the display electrodes when the display state is switched, is minimized. The thickness of the diphthalocyanine films used in the types of electrochromic displays disclosed in the aforementioned patent are typically in the range of 0.2 to 1.0 micron, with the thicker films being used to intensify the color produced by the displays. Such relatively thin films have made it possible to attain switching times of less than 50 milliseconds for display areas on the order of five square centimeters. Of course, it remains desirable, for many visual display applications, to seek further significant improvements of the switching time for display cells.
It is also known that the useful life of an electrochromic display device utilizing diphthalocyanine film can be increased by incorporating into the film, after it is vacuum deposited on a display electrode, a transparent porous binder such as cellulose nitrate or a suitable polycarbonate resin. The binder may be applied by contacting the electrochromic film with a solution of the binder in an organic solvent such as methyl isolbutyl ketone. It would appear that such life protecting resin binders may have some disadvantageous side effects. For example, the binder increases the overall electrochromic film thickness, and thereby possibly increases the switching time of the device from one display state to a second state. There is a recognized correlation between film thickness and visual display switching speed, just as there is a recognized correlation between color contrast or intensity resulting from variation of thickness of the film on the display electrode.
In another prior art patent; U.S. Pat. No. 4,304,465, which issued Dec. 8, 1981, such design parameters are discussed in connection with the disclosure in that patent of an electrochromic display device having a polymer film disposed on its display electrodes to afford visual write and erase functions as a consequence of redox reactions being caused by the controlled passage of an electric charge between a display electrode and a counter electrode within the device. The polymer film used in that disclosed display device is described as being between 0.01 micron to 5 microns thick, with a preferred thickness for the film being 0.05 to 1 micron. In operation of the device the polymer film is oxidized to a colored state to effect a writing step, and then is reduced to a neutral or transparent form to effect an erasing step. Suitable polymers for practicing the invention disclosed in that patent are polyaniline, polypyrrole, and polynaphthylamine. Also, structurally modified substituents of the repeating monomer are said to be suitable for such films; for example, a suitable modified polypyrrole film would be N-phenyl polypyrrole or N-methyl polypyrrole, as disclosed in the patent. Although such polymer films are believed to have desirably long life expectancies, their applications in visual display devices are limited in several important respects. The switching times of such polymer film electrochromic display devices are typically within 0.1 second, which is too slow for many desired display applications. Moreover, such electrochromic polymer film display devices are only capable of being switched between two display states, i.e., a non-transparent or colored state and a transparent state. Thus, there appears to be no possibility of obtaining a range of different colors in a display utilizing only such types of polymer films as those disclosed in that patent. In addition, the color intensity and demarcation between the display colors and backgrounds in the two display states achieved with such polymer film covered display electrodes is often not as pronounced as desired for many display applications.